Adjustable Main Jet and another carb Q

[Marty Levenson]

I have a MMI rebuilt engine that is just over three years old. It has had light use, only ethanol free gas. Been running SO perfectly! Idles nicely at 650 RPM. Haven't touched timing or adjustment since it came from Moyer. FWC (180 degrees) and electronic ignition. The only anomaly, I've just discovered, is sooty plugs.

A few days ago she almost stalled at about 800 RPM as I was about to pick up the mooring. Revved her up and completed the maneuver, but was suspicious of a problem. Took the polishing filter off, and found I could barely blow through it. I was so preoccupied with other issues this spring that I had neglected to change that filter, as well as the Raycor. Thought that might be the problem, but testing after those changes she still wanted to stall below 1000 RPM. Thought the dirty filters might have led to a dirty carb, so I swapped in my spare carb, and indeed she runs fine with my old spare. BUT to my surprise, upon opening up the carb that came with the Moyer rebuild, it is absolutely spotless inside. Like new. Could the idle jets still be dirty and plugged? Seems the only explanation, but incompatible with the state of the carb. Am I missing something?

Tangentially, I have (but not installed) an adjustable main jet (from yet another carb!). Not the mixture adjustment screw (that came installed), but the one that screws in horizontally at the rear of the float chamber. What is the advantage of installing that? How is it adjusted?

Before folks jump on me, yes, I know that I am only managing an upstream problem with dirty gas! I did drain and clean the tank five years ago and have found that diligence with filters has given me reliable power.

Many thanks, in advance.

[JOHN COOKSON]

How long has it been since you serviced the advance? (Under the breaker plate in the distributor)

TRUE GRIT

[Marty Levenson]

advance

Never thought of checking that on my "new" (three-year-old) engine. If that were the problem wouldn't the spare carb have the same idle issue? Or are you suggesting that can account for sooty plugs?

[Marian Claire]

What is the advantage of installing that? How is it adjusted?

My understanding is that the main jet adjustment allows you to control the flow of fuel available to be mixed with the air and delivered to the cylinders. Basically does the same thing the idle mixture adjustment does but the idle adjustment controls air flow and the main adjustment controls fuel.
I have used my main jet to maximize fuel efficiency. With the engine warm and running at "cruising RPM" I adjust, in reduces flow/out increases flow, to get the best performance. I may be sacrificing some performance at other RPM but have noticed no ill effects from this method. By running on the lean side I believe it helps keep the plugs clean and limits excess fuel from passing thru and fouling the water.
There is a lock nut that has to be loosened before turning the T handle to make adjustments. The biggest problem I have had is when re-tightening the locking nut the T may move slightly. Good access is key.
Dan S/V Marian Claire

[roadnsky]

BUT to my surprise, upon opening up the carb that came with the Moyer rebuild, it is absolutely spotless inside.

Marty-
Did you actually "split" the carb open and eyeball the jets?
Back when my carb was a couple of years old, I was surprised when I found a couple of the jets were dirty.

There is a lock nut that has to be loosened before turning the T handle to make adjustments...

I find keeping a light counter leverage on the T-Handle while tightening the nut is important.
As Dan says, you need good access to get back in there.

[JOHN COOKSON]

Never thought of checking that on my "new" (three-year-old) engine. If that were the problem wouldn't the spare carb have the same idle issue? Or are you suggesting that can account for sooty plugs?

Well, I don't know. Maybe the advance was sticking and not sticking. We could go on and on like this.
For me servicing the advance is a yearly maintenance item. Maybe it is time to have a look see at the advance to rule it in or out as a contributing factor? You will be surprised how much better your engine runs.

TRUE GRIT

[Marty Levenson]

advance

Thanks, John - I will check and lube that this fall.

[Marty Levenson]

jets

Thanks, Roadnsky - Yes I did fully disassemble the carb for inspection.

Can one tell by looking if jets are clogged? They LOOK new. I am planning to clean with thin wires and compressed air anyway. Is it necessary to remove the jets for cleaning? I am hesitant as I once destroyed one trying to unscrew it.

[Marty Levenson]

Adjustable main jet

Thanks, Dan - appreciate the clear description. So, to adjust, warmed up at a cruising RPM you slowly tighten the T handle until any change in sound or RPM and then back off a 1/4 turn?

[Ken Rockwell]

Are you getting any black smoke?

[Marian Claire]

Thanks, Dan - appreciate the clear description. So, to adjust, warmed up at a cruising RPM you slowly tighten the T handle until any change in sound or RPM and then back off a 1/4 turn?

I adjust in and out to get the best "sound". What I am looking for is max performance at a set RPM. I determined my "cruising RPM" by taking into account my speed over ground, load on the engine using a vacuum gauge and just how the boat and A-4 sounded. This way I know that when I set my throttle to give me, 2200 RPM in my case, that I am getting good performance. I tend to run for hours at a set RPM.
Dan S/V Marian Claire

[Marty Levenson]

carb

Thanks, Ken, - no smoke.

[Ken Rockwell]

Are the plugs dry or wet?

[Marty Levenson]

plugs

slightly sticky.

[edwardc]

Following in Hanley's experience, I installed an oxygen sensor and a fuel:air ratio gauge, and use it to set the main jet mixture screw for a perfect 14.7:1 ratio at cruise RPM.